= Solution
For an <ideal gas> of fixed molecular composition, translational <energy> is supplemented by rotational, vibrational and eventually <electronic excitations> as their level spacings become thermally accessible. Activating more internal degrees of freedom generally increases the <heat capacity>. In a harmonic vibrational mode the contribution rises from nearly zero when $k_BT\ll\hbar\omega$ to its <classical limit> when $k_BT\gg\hbar\omega$; summing modes with different frequencies gives a smooth increase over an intermediate <temperature> range.
A <monatomic gas> has no <molecular rotations> or vibrations, so its <mass>-specific $c_p$ is nearly constant until <electronic excitation> or <ionization> becomes important: $B$ is then small. A simple <diatomic gas> can also have a small slope when its rotations are already active but its vibrational modes remain frozen. Large flexible polyatomic molecules have many internal modes, often with relatively low excitation energies, and can give a larger slope over their activation range.
The linear law is a local approximation, not an asymptotic result. A finite set of modes approaches finite classical heat-capacity contributions, whereas $A+BT$ grows without bound. At still higher <temperatures> dissociation and <ionization> change the composition and gas constant, invalidating the fixed-molecule ideal-gas model as well. The constants have units $[A]=\mathrm{J\,kg^{-1}\,K^{-1}}$ and $[B]=\mathrm{J\,kg^{-1}\,K^{-2}}$.
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